Entanglement in eight-qubit graph states

En anglemen in eigh -qubi g aph s a es Adán Cabello a,∗, An onio J. López-Ta ida a, Pila Mo eno a, José R. Po illo b a Depa amen o de Física Aplicada II, Uni e sidad de Se illa, E-41012 Se illa, Spain b Depa amen o de Ma emá ica Aplicada I, Uni e sidad de Se illa, E-41012 Se illa, Spain abs ac Any 8-qubi g aph s a e belongs o one o he 101 equi alence classes unde local uni a y ope a ions wi hin he Cli o d g oup. Fo each o hese classes we ob ain a ep esen a i e which equi es he minimum numbe o con olled-Z ga es o i s p epa a ion, and calcula e he Schmid measu e o he 8-pa i e spli , and he Schmid anks o all bipa i e spli s. This esul s in o an ex ension o 8 qubi s o he classifica ion o g aph s a es p oposed by Hein, Eise , and B iegel [M. Hein, J. Eise , H.J. B iegel, Phys. Re . A 69 (2004) 062311]. 1. In oduc ion G aph s a es [1,2] a e a ype o n-qubi pu e s a es ha play se e al undamen al oles in quan um in o ma ion heo y. In quan- um e o -co ec ion, he s abilize codes which p o ec quan um sys ems om e o s [3] can be ealized as g aph codes [4,5].In measu emen -based (o one-way) quan um compu a ion [6],g aph s a es a e he ini ial esou ces consumed du ing he compu a ion. Mo eo e , some g aph s a es a e uni e sal esou ces o quan um compu a ion [7]. In quan um simula ion, g aph s a es allow us o demons a e ac ional b aiding s a is ics o anyons in an exac ly sol able spin model [8]. G aph s a es ha e been used in mul i- pa i e pu ifica ion schemes [9]. The Cli o d g oup has been used o en anglemen dis illa ion p o ocols [10]. G aph s a es na u ally lead o G eenbe ge –Ho ne–Zeilinge (GHZ) o all- e sus-no hing p oo s o Bell’s heo em [11–16], which can be con e ed in o Bell inequali ies which a e maximally iola ed by g aph s a es [17–22]. Some specific g aph s a es a e essen ial o se e al quan um com- munica ion p o ocols, including en anglemen -based quan um key dis ibu ion [23], elepo a ion [24], educ ion o communica ion complexi y [25], and sec e sha ing [26,27]. In addi ion o all hese applica ions, g aph s a es also play a undamen al ole in he heo y o en anglemen . Fo n⩾4qubi s, he e is an infini e amoun o di e en , inequi alen classes o n-qubi pu e en angled s a es. The g aph s a e o malism is a use- *Co esponding au ho . E-mail add ess: [email p o ec ed] (A. Cabello). ul abs ac ion which pe mi s a de ailed (al hough no exhaus i e) classifica ion o n-qubi en anglemen o n⩾4qubi s. Fo all hese easons, a significan expe imen al e o is de- o ed o he c ea ion and es ing o g aph s a es o an inc easing numbe o qubi s. On one hand, he e a e expe imen s o n-qubi n-pho on g aph s a es up o n=6[28–32]. On he o he hand, he combina ion o wo echniques, hype -en anglemen (i.e., en angle- men in se e al deg ees o eedom, like pola iza ion and linea momen um) [33–39] and he sou ces o 4, 5, and 6-pho on en an- glemen using pa ame ic down-con e sion [29,40–45] allows us o c ea e 6-qubi 4-pho on g aph s a es [46,47],8-qubi 4-pho on g aph s a es [46], and e en 10-qubi 5-pho on g aph s a es [46]. The use o 4-pho on sou ces o p epa ing 8-qubi g aph s a es is pa icula ly sui able due o he high isibili y o he esul ing s a es. The classifica ion and s udy o he en anglemen p ope ies o g aph s a es ha e been achie ed, up o 7 qubi s, by Hein, Eise , and B iegel (HEB) in [1] (see also [2]). This classifica ion has been use ul o iden i y new wo-obse e all- e sus-no hing p oo s [16], new Bell inequali ies [21,22], and has s imula ed he p epa a ion o se e al g aph s a es [46].Themainpu poseo hisLe e is o ex end he classifica ion in [1,2] o 8-qubi g aph s a es. Up o 7 qubi s, he e a e 45 classes o g aph s a es ha a e no equi alen unde one-qubi uni a y ans o ma ions. Wi h 8 qubi s, he e a e 101 new classes. All hese classes ha e been ob ained by a ious esea che s (see, e.g., [48]). The pu pose he e is o classi y hem acco ding o se e al ele an physical p ope ies o quan um in o ma ion heo y. The Le e is o ganized as ollows. In Sec ion 2we define qubi g aph s a es and local complemen a ion, which is he main clas- si ying ool. To es ablish an o de be ween he equi alence classes we will use he c i e ia p oposed in [1,2]. These c i e ia a e in o- duced in Sec ion 3. In Sec ion 4we p esen ou esul s. In Sec ion 5 we p esen he conclusions and poin ou some pending p oblems. 2. Basic concep s 2.1. G aph s a e An-qubi g aph s a e |Gis a pu e s a e associa ed o a g aph G=(V,E)consis ing o a se Vo n e ices and a se Eo edges connec ing some o he e ices. Each e ex ep esen s a qubi . The g aph Gp o ides bo h a ecipe o p epa ing |Gand a ma h- ema ical cha ac e iza ion o |G. The ecipe o p epa ing he s a e is he ollowing. Fi s , p e- pa e each qubi in he s a e |+ = (|0+|1)/√2. Then, o each edge connec ing wo qubi s, iand j, apply he con olled-Zga e be ween qubi s iand j, i.e., he uni a y ans o ma ion CZ= |0000|+|0101|+|1010|−|1111|. The ma hema ical cha ac e iza ion o |Gis he ollowing. The g aph s a e |Gassocia ed o he g aph Gis he only n-qubi s a e which ulfills gi|G=|G, o i=1,...,n,(1) whe e gia e he gene a o s o he s abilize g oup o he s a e, defined as he se {sj}2n j=1o all p oduc s o he gene a o s. Speci - ically, giis he gene a o ope a o associa ed o he e ex i,de- fined by gi:= X(i) j∈N(i) Z(j),(2) whe e N(i)is he neighbo hood o he e ex i, i.e., hose e ices which a e connec ed o i, and X(i)(Z(i)) deno es he Pauli ma ix σx(σz)ac ingon hei h qubi . 2.2. Local complemen a ion Fo ou pu poses, he key poin is ha local complemen a ion (LC) is a simple ans o ma ion which lea es he en anglemen p ope ies in a ian . Two n-qubi s a es, |φand |ψha e he same n-pa i e en an- glemen i and only i he e a e none-qubi uni a y ans o ma- ions Ui, such ha |φ=n i=1Ui|ψ. I hese one-qubi uni a y ans o ma ions belong o he Cli o d g oup, hen bo h s a es a e said o be local Cli o d equi alen . The one-qubi Cli o d g oup is gene a ed by he Hadama d ga e H=(|00|+|01|+|10|− |11|)/√2 and he phase ga e P=|00|+i|11|. Van den Nes , Dehaene, and De Moo ound ha he succes- si e applica ion o a ans o ma ion wi h a simple g aphical de- sc ip ion is sufficien o gene a e he comple e equi alence class o g aph s a es unde local uni a y ope a ions wi hin he Cli o d g oup (he ea e simply e e ed as class o o bi ) [49].Thissimple ans o ma ion is LC. On he s abilize , LC on he qubi iinduces he map Y(i)→ Z(i),Z(i)→ −Y(i)on he qubi i, and he map X(j)→ −Y(j), Y(j)→ X(j)on he qubi s j∈N(i)[2]. On he gene a o s, LC on he qubi imaps he gene a o s gold jwi h j∈N(i) o gnew jgnew i. G aphically, LC on he qubi iac s as ollows: One picks ou he e ex iand in e s he neighbo hood N(i)o i; i.e., e ices in he neighbo hood which we e connec ed become disconnec ed and ice e sa. I has been shown by Van den Nes , Dehaene, and De Moo ha o a pa icula class o qubi g aph s a es local uni a y equi alence implies local Cli o d equi alence [50]. Mo eo e , nume ical e- sul s show ha local Cli o d equi alence coincides wi h local uni- a y equi alence o qubi g aph s a es associa ed wi h connec ed g aphs up o n=7 e ices[1,2]. I should be no ed, howe e , ha no all local uni a y ans o ma ions be ween g aph s a es can be ep esen ed as successi e LCs. A coun e example wi h n=27 is de- sc ibed in [51]. Using LC, one can gene a e he o bi s o all LC-inequi alen n- qubi g aph s a es. Fo a small n, he numbe o o bi s has been well known o a long ime (see, e.g., [48]). Specifically, o n=8, he e a e 101 LC-inequi alen classes. 3. C i e ia o he classifica ion Following HEB, he c i e ia o o de ing he classes a e: (a) numbe o qubi s, (b) minimum numbe o con olled-Zga es needed o he p epa a ion, (c) he Schmid measu e, and (d) he ank indexes. Fo ins ance, class No. 1 is he only one con ain- ing wo-qubi g aph s a es, class No. 2 is he only one con aining h ee-qubi g aph s a es [1,2]. Classes No. 3 and No. 4 bo h ha e n=4 qubi s and equi e a minimum o |E|=3 con olled-Zga es. Howe e , class No. 3 has Schmid measu e ES=1, while class No. 4 has ES=2. 3.1. Minimum numbe o con olled-Z ga es o he p epa a ion Di e en membe s o he same LC class equi e a di e en numbe o con olled-Zga es o hei p epa a ion s a ing om he s a e |+=(|0+|1)/√2 o each qubi . The fi s c i e ion o ou classifica ion is he minimum numbe o con olled-Zga es equi ed o p epa ing one g aph s a e wi hin he LC class. This co - esponds o he numbe o edges o he g aph wi h he minimum numbe o edges wi hin he LC class, |E|. We will p o ide a ep e- sen a i e wi h he minimum numbe o edges o each LC class. 3.2. Schmid measu e The Schmid measu e was in oduced by Eise and B iegel as a ool o quan i ying he genuine mul ipa i e en anglemen o quan um sys ems [52] (see also [53]). Any s a e ec o |ψ∈ H(1)⊗···⊗H(N)o a composi e quan um sys em wi h Ncom- ponen s can be ep esen ed as |ψ= R  i=1 ξiψ(1) i⊗···⊗ψ(N) i,(3) whe e ξi∈C o i=1,...,R, and |ψ(j) i∈H(j), o j=1,...,N. The Schmid measu e associa ed wi h a s a e ec o |ψis hen defined as ES|ψ =log2( ), (4) whe e is he minimal numbe Ro e msin hesumo Eq.(3) o e all linea decomposi ions in o p oduc s a es. In case o a wo- componen sys em (N=2), he minimal numbe o p oduc e ms is gi en by he Schmid ank o he s a e |ψ. Hence, he Schmid measu e could be conside ed a gene aliza ion o he Schmid ank o mul ipa i e quan um sys ems [see Eq. (9) below]. The Schmid measu e can be ex ended o mixed s a es by means o a con ex oo ex ension. In his Le e , howe e , we will deal only wi h pu e s a es. Gi en a g aph G=(V,E),apa i ion o Vis any uple (A1,...,AM)o disjoin subse s Ai⊂V,wi hM i=1Ai=V.Incase M=2, we e e o he pa i ion as a bipa i ion, and deno e i (A,B).Wewillw i e (A1,...,AN)⩽(B1,...,BM), (5) i (A1,...,AN)is a fine pa i ion han (B1,...,BM), which means ha e e y Aiis con ained in some Bj. The la e is hen a coa se pa i ion han he o me . Fo any g aph G=(V,E), hepa - i ioning whe e (A1,...,AM)=Vsuch ha |Ai|=1, o e e y i=1,...,M, is e e ed o as he fines pa i ion. We mus poin ou ha ESis noninc easing unde a coa se g aining o he pa i ioning: I wo componen s a e me ged in o - de o o m a new componen , hen he Schmid measu e can only dec ease. I we deno e he Schmid measu e o a s a e ec- o |ψe alua edwi h espec oapa i ioning(A1,...,AN)as E(A1,...,AN) S(|ψ), meaning ha he espec i e Hilbe spaces a e hose o he g ains o he pa i ioning, hen he noninc easing p ope y o EScan be exp essed as E(A1,...,AN) S|ψ⩾E(B1,...,BM) S|ψ,(6) i (A1,...,AN)⩽(B1,...,BM). Le (A,B)be a bipa i ion (i.e., A∪B=V;A∩B=∅) o a g aph G=(V,E),wi hV={1,...,N}, and le us deno e he adjacency ma ix o he g aph by Γ, i.e., he symme ic ma ix wi h elemen s Γij =1,i (i,j)∈E, 0,o he wise. (7) When we a e dealing wi h a bipa i ion, i is use ul o label he e ices o he g aph so ha A={1,...,p},B={p+1,...,N}. Then, we can decompose he adjacency ma ix Γin o subma i- ces ΓA,ΓB( ha ep esen edges wi hin Aand edges wi hin B), and ΓAB ( he |A|×|B|o -diagonal subma ix o he adjacency ma ix Γ ha ep esen s hose edges be ween Aand B), ΓAΓAB ΓT AB ΓB=Γ. (8) The Schmid ank SRA(G)o a g aph s a e |G ep esen ed by he g aph G=(V,E), wi h espec o he bipa i ion (A,B),isgi enby he bina y ank [i.e., he ank o e GF(2)] o he subma ix ΓAB, SRA(G)= ankF2(ΓAB). (9) I ollows s aigh o wa dly om he defini ion ha SRA(G)= SRB(G), because he di e en bipa i ions a e fixed by choosing he smalle pa , say A, o he bipa i ion (A,B), which gi es 2N−1bi- pa i ions. 3.3. Rank indexes While calcula ing he Schmid ank wi h espec o all possible bipa i ions o a gi en g aph, le us coun how many imes a ce - ain ank occu s in all he bipa i e spli s, and hen classi y his in o ma ion acco ding o he numbe o e ices in A, he smalle pa o he spli unde conside a ion. The e is a compac way o exp ess his in o ma ion, he so-called ank indexes [1,2]. The ank index o all he bipa i e spli s wi h p e ices in he smalle pa Ais gi en by he p- uple RIp=νp p,...,νp 1=νp j1 j=p,(10) whe e νp jis he numbe o imes in which SRA(G)=j,wi h |A|=p, occu s. 4. P ocedu es and esul s The main esul s o he Le e a e summa ized in Fig. 2 and Table 1. In he ollowing, we p o ide de ails on he calcula ions leading o hese esul s. 4.1. O bi s unde local complemen a ion We ha e gene a ed all LC o bi s o n=8 and calcula ed he numbe o non-isomo phic g aphs in each LC o bi , deno ed by |LC|. These numbe s a e coun ed up o isomo phism. Fig. 1. The se o e ices 4, 6, and 8 is he minimal e ex co e o he g aph (le ). The se o e ices 3, 5, 7, and 8, is a e ex co e o he g aph, bu is no minimal, i has size 4 ( igh ). In addi ion, o each o bi , we ha e calcula ed a ep esen a- i e wi h he minimum numbe o edges |E|. As ep esen a i e, we ha e chosen he one (o one o hose) wi h he minimum numbe o edges and he minimum maximum deg ee (i.e., numbe o edges inciden wi h a e ex). This means ha he g aph s a e associ- a ed o his g aph equi es he minimum numbe o con olled-Z ga es o i s p epa a ion, and he minimum p epa a ion dep h (i.e., i s p epa a ion equi es a minimum numbe o s eps) [54].All he ep esen a i es o each o he 101 o bi s a e illus a ed in Fig. 2. |LC|and |E|a e in Table 1. 4.2. Bounds o he Schmid measu e I is a well-known ac ha o any measu e o mul ipa icle en anglemen p oposed so a , including he Schmid measu e ES, he compu a ion is exceedingly difficul o gene al s a es. In o de o de e mine ES, one has o show ha a gi en decomposi ion in Eq. (3) wi h R e ms is minimal. Fo a gene al s a e, he minimiza- ion p oblem in ol ed can be a e y difficul p oblem o nume ical analysis, which scales exponen ially in he numbe o pa ies Nas well as in he deg ee o en anglemen o he s a e i sel . Ne e he- less, his ask becomes easible i we es ic ou a en ion o g aph s a es. HEB es ablished se e al uppe and lowe bounds o he Schmid measu e in g aph heo e ical e ms [1,2]. These bounds make possible o de e mine he Schmid measu e o a la ge num- be o g aphs o p ac ical impo ance, because in many cases he bounds p oposed a e easily compu able and, ema kably, he uppe and lowe bounds equen ly coincide. 4.2.1. Pauli pe sis ency and size o he minimal e ex co e Fo any g aph s a e |G, uppe bounds o i s Schmid measu e ES(|G)a e he Pauli pe sis ency PP(G)and he size o he minimal e ex co e VC(G), ES|G⩽PP(G)⩽VC(G). (11) The Pauli pe sis ency is he minimal numbe o local Pauli mea- su emen s necessa y o disen angle a g aph s a e. Conce ning his ques ion, HEB desc ibed g aphical ans o ma ion ules when local Pauli measu emen s a e applied [1,2]. A e ex co e is a concep om g aph heo y: I is any subse V⊆Vo e ices in a g aph G o which any edge o Gis inci- den (see Fig. 1). The e o e, he minimal e ex co e o a g aph is he smalles one, whose size is deno ed by VC(G). Acco ding o he g aphical ules o he Pauli measu emen s, since each σzmeasu e- men simply dele es all edges inciden o a e ex, he size o he minimal e ex co e would equal he Pauli pe sis ency, p o ided ha we es ic he Pauli measu emen s o σzmeasu emen s. Ne - e heless, in g aphs wi h many edges, i.e., e y connec ed, a p ope combina ion o σx,σy, and σzmeasu emen s could p o ide a mo e efficien disen angling sequence, gi ing a be e uppe bound PP(G) o he Schmid measu e. See [1,2] o de ails. 4.2.2. Maximal Schmid ank Fo any g aph s a e |G, a lowe bound o he Schmid measu e ES(|G)is he maximal Schmid ank, SRmax(G)⩽ES|G.(12) While calcula ing he Schmid ank wi h espec o all possible bipa i ions o a gi en g aph G=(V,E), i one maximizes he Schmid ank o e all bipa i ions (A,B)o he g aph, and akes in o accoun he noninc easing p ope y o ES|(G)[see Eq. (6)], hen one ob ains a lowe bound o he Schmid measu e wi h e- spec o he fines pa i ioning. This lowe bound is he maximal Schmid ank, SRmax(G):=max A⊆VSRA(G). (13) Acco ding o he defini ion o Schmid ank, he maximal Schmid ank o any s a e is, a mos , N 2, i.e., he la ges in ege less han o equal o N 2. 4.2.3. Addi ion and dele ion o edges and e ices As we men ioned in Sec ion 2.2, applying he LC- ule does no change he Schmid measu e ES. I is in e es ing o ema k ha o he local changes o he g aph, such as he dele ion o edges o e ices, ha e only a limi ed e ec on ES. This ac is es ablished by HEB [1] in wha hey call he edge/ e ex ule: On one hand, by dele ing (o adding) an edge ebe ween wo e ices o a g aph G he Schmid measu e o he esul ing g aph G=G±ecan a mos dec ease (o inc ease) by 1. On he o he hand, i a e ex (including all i s inciden edges) is dele ed, he Schmid measu e o he esul ing g aph G=G− canno inc ease, and will a mos dec ease by one. I ES(|G+e)deno es he Schmid measu e o he g aph s a e co esponding o he g aph G+e, hen he p e ious ules can be summa ized as ES|G+e⩽ES|G+1,(14a) ES|G−e⩾ES|G−1,(14b) ES|G− ⩾ES|G −1.(14c) We ha e used hese ules in wo ways: Fi s ly, as an in e nal es o check ou calcula ions, compa ing pai s o g aphs connec ed by a sequence o addi ion o dele ion o edges/ e ices; and secondly, as a use ul ool ha , in some g aphs, has enabled us o go om a bounded o a de e mined alue o he Schmid measu e, once again by compa ison be ween a p oblema ic g aph Gand a e- sul ing g aph G( ypically ob ained by edge o e ex dele ion) o a known Schmid measu e. 4.2.4. Schmid measu e in some special ypes o g aphs The e a e some special ypes o g aph s a es in which lowe and uppe bounds o he Schmid measu e coincide (see [1]), gi - ing di ec ly a de e mined alue o ES. Since he maximal Schmid ank o any s a e can be a mos N 2, and es ic ing ou sel es o s a es wi h coinciden uppe and lowe bounds o ES,i is ue ha SRmax(G)=ES(|G)=PP(G)=VC(G)⩽N 2. This is he case o GHZ s a es, and s a es ep esen ed by ees, ings wi h an e en numbe o e ices, and clus e s. In ou wo k we ha e used he ollowing esul s conce ning GHZ s a es and ees: (a) The Schmid measu e o any mul ipa i e GHZ s a e is 1. (b) A ee T is a g aph ha has no cycles. The Schmid measu e o he co esponding g aph s a e |Tis he size o i s minimal e ex co e : ES(|T)=VC(T). The e is ano he in e es ing kind o g aphs o ou pu poses, he so-called 2-colo able g aphs. A g aph is said o be 2-colo able when i is possible o pe o m a p ope 2-colo ing oni :Thisisalabeling V→{1,2}, such ha all connec ed e ices a e associa ed wi h a di e en elemen om {1,2}, which can be iden ified wi h wo colo s. I is a well-known ac in g aph heo y ha a necessa y and sufficien c i e ion o a g aph o be 2-colo able is ha i does no con ain any cycles o odd leng h. Ma hema icians call hese g aphs bipa i e g aphs due o he ac ha he whole se o e ices can be dis ibu ed in o wo disjoin subse s Aand B, such ha no wo e ices wi hin he same subse a e connec ed, and he e o e e e y edge connec s a e ex in Awi h a e ex in B. HEB [1] p o ided lowe and uppe bounds o he Schmid measu e ha could be applied o g aph s a es ep esen ed by 2- colo able g aphs: 1 2 ankF2(Γ ) ⩽ES|G⩽|V| 2,(15) whe e Γis he adjacency ma ix o he 2-colo able g aph. I Γis in e ible, hen ES|G =|V| 2.(16) Besides, HEB poin ed ou ha any g aph Gwhich is no 2-co- lo able can be u ned in o a 2-colo able one Gby simply dele ing he app op ia e e ices on cycles wi h odd leng h p esen in G. The iden ifica ion o his g aphical ac ion wi h he e ec o a σz measu emen on qubi s co esponding o such e ices yields new uppe bounds o ES(|G): ES|G⩽ES|G+M⩽|V−M| 2+M⩽|V|+M 2,(17) whe e Mis henumbe o emo ed e ices.Weha eused hese new bounds in some g aphs as a ool o check ou calcula ions. 5. Conclusions, open p oblems, and u u e de elopmen s To sum i all up, we ha e ex ended o 8 qubi s he classifica- ion o he en anglemen o g aph s a es p oposed in [1] o n<8 qubi s. No ice ha o n=8 we ha e 101 classes, while o n<8 he e a e only 45 classes. Fo each o hese classes we ob ain a ep esen a i e which equi es he minimum con olled-Zga es o i s p epa a ion (see Fig. 2), and calcula e he Schmid measu e o he 8-pa i e spli (which measu es he genuine 8-pa y en angle- men o he class), and he Schmid anks o all bipa i e spli s (see Table 1). This classifica ion will help us o ob ain new all- e sus-no hing p oo s o Bell’s heo em [16] and new Bell inequali ies. Specifically, any 8-qubi g aph s a e belonging o a class wi h a ep esen a i e wi h 7 edges (i.e., a ee) has a specific ype o Bell inequali y [22]. Mo e gene ally, i will help us o in es iga e he nonlocali y (i.e., he non-simulabili y o he p edic ions o quan um mechanics by means o non-local hidden a iable models) o g aph s a es [21]. Ex ending he classifica ion in [1] a u he s ep sheds some ligh on he limi a ions o he me hod o classifica ion. The c i e- ia used in [1] o o de he classes (see Sec ion 3)al eady ailed o dis inguish all classes in n=7. Fo ins ance, classes No. 40, No. 42, and No. 43 in [1,2] ha e he same numbe o qubi s, equi e he same minimum numbe o con olled-Zga es o he p epa a- ion, and ha e he same Schmid measu e and ank indexes. The same p oblem occu s be ween classes No. 110 and No. 111, be- ween classes No. 113 and No. 114, and be ween classes No. 116 and No. 117 in ou classifica ion (see Table 1). Following [1,2],we ha e placed he class wi h lowe |LC|in he fi s place. Howe e , his solu ion is no sa is ac o y, since |LC|is no ela ed o he en- anglemen p ope ies o he class. On he o he hand, Van den Nes , Dehaene, and De Moo ha e p oposed a fini e se o in- a ian s ha cha ac e izes all classes [55]. Howe e , his se has mo e han 2 ×1036 in a ian s al eady o n=7. The p oblem o ob aining a minimum se o in a ian s capable o dis inguishing all classes wi h n⩽8 qubi s will be add essed elsewhe e [56]. Ano he weak poin in he me hod is ha he p ecise alue o ESis s ill unknown o some classes. The good news is ha , o mos o hese classes, he alue migh be fixed i we knew he Fig. 2. G aphs associa ed o he 101 classes on 8-qubi g aph s a es inequi alen unde local complemen a ion and g aph isomo phism. We ha e chosen as ep esen a i e o he class he one (o one o hose) wi h minimum numbe o edges and minimum maximum deg ee (i.e., numbe o edges inciden wi h a e ex), which means ha i equi es he minimum numbe o con olled-Zga es in he p epa a ion and minimum p epa a ion dep h. Table 1 En anglemen o he 101 classes o 8-qubi g aph s a es. No. is he numbe o he class; i is assigned a ending o |E|,ES,andRI j; he numbe ing s a s a he poin in which he one in Re s. [1,2] ends. |LC|is he numbe o nonisomo phic elemen s o he class. |E|is he numbe o edges o hose ep esen a i es wi h he minimum numbe o edges. ESis he Schmid measu e (o i s lowe and uppe bounds). RI jis he ank index wi h jqubi s in he smalle bipa i ion (i.e., he numbe o bipa i e spli s in which a ce ain ank occu s; anks appea in dec easing o de om le o igh ). 2-col indica es whe he o no a 2-colo able ep esen a i e exis s. No. |LC||E|ESRI4RI3RI22-col No. |LC||E|ESRI4RI3RI22-col 46 2 7 1 (0,0,0,35) (0,0,56) (0,28) yes 97 154 8 4 (8,22,4,1) (42,13,1) (27,1) no 47 6 7 2 (0,0,20,15) (0,30,26) (12,16) yes 98 542 8 4 (8,22,5,0) (42,14,0) (27,1) no 48 6 7 2 (0,0,30,5) (0,45,11) (15,13) yes 99 300 8 4 (12,16,7,0) (44,11,1) (27,1) yes 49 16 7 2 (0,0,30,5) (0,45,11) (17,11) yes 100 214 8 4 (14,17,4,0) (48,8,0) (28,0) yes 50 4 7 2 (0,0,34,1) (0,48,8) (16,12) yes 101 14 9 3 (0,20,15,0) (24,32,0) (25,3) no 51 16 7 2 (0,0,34,1) (0,51,5) (19,9) yes 102 66 9 3 (0,28,7,0) (32,24,0) (25,3) no 52 10 7 3 (0,12,16,7) (16,28,12) (20,8) yes 103 66 9 3 (0,30,5,0) (36,20,0) (26,2) yes 53 16 7 3 (0,12,22,1) (16,34,6) (20,8) yes 104 6 9 3 (0,32,0,3) (32,24,0) (24,4) yes 54 44 7 3 (0,12,22,1) (16,35,5) (21,7) yes 105 57 9 3 <4 (0,30,5,0) (36,19,1) (25,3) no 55 16 7 3 (0,18,14,3) (18,33,5) (21,7) yes 106 28 9 4 (8,18,9,0) (38,18,0) (25,3) no 56 44 7 3 (0,18,14,3) (22,29,5) (23,5) yes 107 17 9 4 (8,20,6,1) (32,24,0) (24,4) no 57 10 7 3 (0,18,15,2) (18,36,2) (21,7) yes 108 72 9 4 (8,20,7,0) (36,20,0) (25,3) no 58 25 7 3 (0,18,16,1) (18,36,2) (22,6) yes 109 87 9 4 (8,20,7,0) (40,16,0) (27,1) no 59 44 7 3 (0,18,16,1) (22,31,3) (23,5) yes 110 114 9 4 (8,22,5,0) (40,16,0) (26,2) no 60 44 7 3 (0,24,9,2) (24,30,2) (23,5) yes 111 372 9 4 (8,22,5,0) (40,16,0) (26,2) no 61 26 7 3 (0,24,10,1) (28,25,3) (23,5) yes 112 70 9 4 (8,24,2,1) (40,16,0) (26,2) no 62 120 7 3 (0,24,10,1) (28,26,2) (24,4) yes 113 264 9 4 (8,24,3,0) (44,12,0) (27,1) no 63 66 7 3 (0,26,7,2) (30,24,2) (25,3) yes 114 542 9 4 (8,24,3,0) (44,12,0) (27,1) no 64 14 7 4 (8,12,12,3) (32,18,6) (24,4) yes 115 156 9 4 (12,18,5,0) (46,9,1) (27,1) no 65 25 7 4 (8,12,14,1) (32,20,4) (24,4) yes 116 174 9 4 (12,20,3,0) (46,10,0) (27,1) no 66 120 7 4 (8,14,12,1) (34,19,3) (25,3) yes 117 542 9 4 (12,20,3,0) (46,10,0) (27,1) no 67 72 7 4 (8,16,10,1) (36,17,3) (25,3) yes 118 262 9 4 (12,20,3,0) (48,8,0) (28,0) no 68 172 7 4 (8,18,8,1) (38,16,2) (26,2) yes 119 802 9 4 (14,19,2,0) (50,6,0) (28,0) no 69 10 8 2 (0,0,34,1) (0,52,4) (20,8) yes 120 117 9 4 (16,16,3,0) (50,6,0) (28,0) yes 70 10 8 2 (0,0,35,0) (0,54,2) (21,7) yes 121 10 10 3 (0,32,2,1) (32,24,0) (24,4) no 71 10 8 3 (0,12,22,1) (16,36,4) (20,8) no 122 37 10 3 (0,32,3,0) (40,16,0) (27,1) yes 72 21 8 3 (0,12,22,1) (16,36,4) (22,6) no 123 36 10 4 (8,22,5,0) (44,12,0) (26,2) no 73 10 8 3 (0,18,17,0) (18,36,2) (21,7) no 124 7 10 4 (8,24,0,3) (32,24,0) (24,4) no 74 44 8 3 (0,18,17,0) (22,32,2) (23,5) yes 125 103 10 4 (8,24,3,0) (42,14,0) (26,2) no 75 66 8 3 (0,18,17,0) (22,33,1) (24,4) no 126 46 10 4 (8,24,3,0) (44,12,0) (27,1) no 76 26 8 3 (0,20,14,1) (24,30,2) (24,4) yes 127 170 10 4 (8,26,1,0) (46,10,0) (27,1) no 77 26 8 3 (0,24,10,1) (24,31,1) (23,5) yes 128 74 10 4 (12,20,3,0) (46,10,0) (27,1) yes 78 28 8 3 (0,24,10,1) (28,27,1) (23,5) no 129 340 10 4 (12,22,1,0) (48,8,0) (27,1) no 79 44 8 3 (0,24,11,0) (28,26,2) (23,5) no 130 254 10 4 (12,22,1,0) (50,6,0) (28,0) no 80 132 8 3 (0,24,11,0) (28,27,1) (24,4) no 131 433 10 4 (14,21,0,0) (52,4,0) (28,0) no 81 114 8 3 (0,24,11,0) (30,25,1) (25,3) yes 132 476 10 4 (16,18,1,0) (52,4,0) (28,0) no 82 72 8 3 (0,26,9,0) (30,26,0) (25,3) no 133 28 10 4 <5 (12,22,0,1) (48,8,0) (28,0) no 83 72 8 3 (0,28,6,1) (32,23,1) (25,3) yes 134 9 11 3 <4 (0,30,5,0) (40,15,1) (25,3) no 84 198 8 3 (0,28,7,0) (34,22,0) (26,2) yes 135 39 11 4 (8,26,1,0) (44,12,0) (26,2) no 85 56 8 3 <4 (0,30,4,1) (34,21,1) (25,3) no 136 46 11 4 (12,20,3,0) (50,6,0) (27,1) no 86 28 8 4 (8,16,10,1) (32,22,2) (24,4) no 137 208 11 4 <5 (16,18,1,0) (52,4,0) (28,0) no 87 10 8 4 (8,16,10,1) (32,24,0) (24,4) yes 138 298 11 4 <5 (18,17,0,0) (54,2,0) (28,0) no 88 56 8 4 (8,16,10,1) (36,18,2) (26,2) no 139 24 11 4 <5 (20,10,5,0) (50,5,1) (27,1) no 89 66 8 4 (8,16,11,0) (36,18,2) (25,3) yes 140 267 11 4 <5 (20,14,1,0) (54,2,0) (28,0) no 90 72 8 4 (8,18,9,0) (34,22,0) (25,3) no 141 4 12 4 (28,0,7,0) (56,0,0) (28,0) no 91 63 8 4 (8,18,9,0) (36,20,0) (26,2) yes 142 22 12 4 <5 (14,21,0,0) (56,0,0) (28,0) no 92 66 8 4 (8,18,9,0) (38,16,2) (25,3) no 143 46 12 4 (20,12,3,0) (50,6,0) (27,1) yes 93 176 8 4 (8,18,9,0) (38,17,1) (26,2) no 144 28 13 4 (28,4,3,0) (56,0,0) (28,0) no 94 76 8 4 (8,20,6,1) (36,19,1) (25,3) no 145 7 13 4 <5 (16,18,1,0) (56,0,0) (28,0) no 95 194 8 4 (8,20,7,0) (38,18,0) (26,2) yes 146 51 13 4 <5 (24,10,1,0) (56,0,0) (28,0) no 96 352 8 4 (8,20,7,0) (40,15,1) (26,2) no alue o he 5-qubi ing clus e s a e, which is he fi s g aph s a e in he classifica ion o which he alue o ESis unknown [1,2]. Un o una ely, we ha e no made any p og ess in calcula ing ES o he 5-qubi ing clus e s a e. Table 1 shows ha he e a e no 8-qubi g aph s a es wi h ank indexes RIp=[νp j]1 j=pwi h νp j= 0i j=p, and νp j=0i j<p, i.e., wi h maximal ank wi h espec o all bipa i e spli s, i.e., such ha en anglemen is symme ically dis ibu ed be ween all pa ies. These s a es a e obus agains disen anglemen by a ew measu e- men s. Nei he he e a e 7-qubi g aph s a es wi h his p ope y [1,2]. This makes mo e in e es ing he ac ha he e is a single 5-qubi and a single 6-qubi g aph s a e wi h his p ope y [1,2]. Acknowledgemen s The au ho s hank H.J. B iegel, O. Gühne, M. G assl, M. Hein, C. San ana, and M. Van den Nes , o hei help. This wo k has benefi ed om he use o he p og am nau y [57] o compu ing au omo phism g oups o g aphs. A.C., A.J.L., and P.M. acknowledge suppo om p ojec s No. P06-FQM-02243, No. FIS2008-05596, and No. PAI-FQM-0239. 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